Operating Tips for Big-Bore Continentals
AVweb editor and powerplant management authority Mike Busch walks you through each phase of a typical mission — preflight, start, taxi, runup, takeoff, climb, cruise, descent, landing and shutdown — and explains how to operate your big-bore Continental engine for maximum longevity and safety. If you fly a Bonanza, Baron, Centurion, Twin Cessna, or anything else powered by a big Continental, you'll want to read this. (Lycoming operators will find a lot of useful guidance here, too.)
Big-bore Continental engines the IO-470, IO-520, IO-550 and their turbocharged counterparts are perhaps the most popular high-performance piston powerplants in General Aviation. These engines power Beech Bonanzas and Barons, Cessna 200-series singles and Cessna 300- and 400-series twins, and many other models. A carburetor-equipped version the O-470 powers the ubiquitous Cessna 182. The turbocharged versions have been especially successful: there are more Continental TSIO-520s flying than all other turbocharged piston engines combined.
All big-bore Continentals share a common design and many common parts. A -520 is simply a -470 that has been fitted with larger cylinders with a quarter-inch larger bore, and a -550 is nothing more than a -520 modified to have a quarter-inch longer stroke. Most parts are interchangeable between these engine families.
The big-bores come in two basic flavors. The engines used in Beech Bonanzas and Barons and in Twin Cessnas use the "Permold" crankcase configuration characterized by a front-mounted gear-driven alternator and a rear-mounted oil cooler. In contrast, the big Cessna singles use the "sandcast" case configuration, recognizable by its rear-mounted belt-driven alternator and front-mounted oil cooler. Although the Permold and sandcast engines differ from each other physically in various important ways, there's not much difference in the way they are flown from the pilot's eye view.
This article focuses primarily on powerplant management techniques for the fuel-injected big-bore engines fitted with constant-speed propellers. However, most of the techniques also apply to the carbureted O-470 (Skylane), and many are applicable to Lycoming-powered high-performance airplanes as well.
So without further ado, let's proceed through the sequential phases of a typical flight from preflight to shutdown, and discuss engine-related pilot techniques that you can employ to assure optimum engine longevity and safety of flight.
Few things make a pilot feel more foolish than to discover a problem in the air that should have been caught on preflight. In my 30 years of flying, I've had my share. I've tried taxing out of a parking space with the tail still tied down (and all my friends watching). I've aborted a takeoff halfway down the runway because I neglected to remove the pitot cover...and with an FAA guy in the right seat. I've taken off with an unlatched cabin door on several occasions. And I've fired up without pulling the chocks more times than I care to remember.
Fortunately, most of those things injured my pride but little else. But some problems can kill you if they're missed at preflight. Chief among these are engine discrepancies. Unlike the rest of the airplane, the powerplant is constantly trying to burn itself up and shake itself apart, and to do the same with everything else nearby.
Consequently, the engine compartment needs to be looked at carefully...and often. Giving it a careful going-over at each 50 hour oil change is simply not enough. Some aircaft notably Bonanzas and Twin Cessnas have big hinged engine compartment doors that allow quick and easy preflight access, and these should definitely be opened up at preflight before the first flight of the day. Other aircraft (including most Cessna singles) make it a lot harder to preflight the engine compartment. But even on these models, I recommend pulling the top cowling at least every ten hours or so.
What should you look for in there? Definitely look for stains of all sorts: fuel, oil, and exhaust. Fuel stains are normally blue (if you use blue-dyed 100LL avgas). Oil stains are, well, oily. Exhaust stains are generally brightly colored: mostly yellow, orange or red. Stains are almost always a tip-off that something's wrong. Even if the stains are small ones, don't fly the airplane until you're sure you know where the stain is coming from.
If you're absolutely certain that small oil leak is coming from a bad rocker cover gasket, it's probably okay to fly now and replace the gasket later. But if it's coming from the a cracked oil cooler, oil filter adapter, or from cylinder, then it's definitely not okay to fly! So make sure you know the source.
Also inspect the engine compartment and nacelle exterior for signs of heat distress. Hidden exhaust leaks can be killers, particularly in turbocharged aircraft. Turbocharged Cessna twins have an especially dismal history of exhaust-related fatal accidents. Also look for signs of chafing where hoses, wire bundles and control cables come into close proximity to the engine or each other. If you see two things rubbing, isolate them with a tie-wrap or clamp before you launch, lest the chafing continue and cause a serious problem in-flight.
Fuel contamination is another killer that needs to be caught at preflight. If the airplane has been out in the rain or fueled from underground tanks, you need to be especially careful about draining all the sumps (including aux tanks and crossfeed lines, if applicable) until all water has been removed. Don't drain the sumps onto the ground...drain them into a jar or fuel tester so you can see what you've got. (I use a "GATS jar" from Sporty's.)
I also recommend sniffing the fuel you drain to see if you can detect the odor of kerosene. My Cessna T310 was once misfueled with Jet A and I'd probably be dead now if I hadn't caught it before flight.
If it's wintertime and you can't get fuel to come out of any drain, the reason may be water contamination that has turned to ice and plugged the drain. Don't fly until the airplane has been thawed and drained thoroughly. Most manufacturers recommend adding anhydrous isopropyl alcohol or ethylene glycol monomethyl ether (EGME, sold as "Prist") to the fuel to prevent fuel system icing. See your POH for details.
Injected Continentals are typically very easy to start. The aux pump and injector nozzles provide optimum priming right at the intake port of each cylinder. Flip on the battery, alternator and mag switch; advance the mixture and prop to full-forward; advance the throttle a bit; then crank while priming with the electric fuel pump. The engine should start within a few blades' rotation. As soon as it does, release the starter and prime switches immediately...especially important if you have a Bendix "Shower of Sparks" ignition system, since the mag timing remains retarded until you release the start switch.
If it's cold out, a bit more priming than usual may be required. Advance the throttle all the way, hit the prime switch until the fuel flow gauge stabilizes, then retard the throttle and crank the engine. If the engine catches and then dies, the fuel has been consumed and you'll have to start the process again. Of course, if it's really cold out (below freezing), you'll want to get a pre-heat before starting; otherwise you risk damaging the engine's top-end components.
Hot starts are the most challenging, and give many pilots absolute fits. Some instructors recommend intentionally flooding the engine, and then cranking it with the mixture at idle cutoff until it starts; I don't care for this technique because it's an engraved invitation for a stack fire.
Instead, I recommend the following hot-start technique which works every time on virtually any engine which uses the TCM continuous-flow injection system: with the throttles full open and the mixtures at idle cutoff, run the electric fuel pump on "high" (or "prime") for a full 30 seconds. This circulates cool fuel through the fuel lines, fuel pump and fuel control unit, purging them of vapor. After a half-minute, turn off the pump, advance the mixtures to full rich, retard the throttles, and crank the engines, using no prime or just a touch.
If you're flying a late-model GTSIO-520 equipped with a "seventh nozzle" there's a special starting procedure; check your POH.
If you have difficulty starting the engine, remember this rule-of-thumb: if the engine is hot, you've probably flooded it; if the engine is cold, you've probably not primed it enough. Also, don't crank the engine for more than about 15 seconds without giving the starter a rest so it can cool off. Starter motors are not rated for continuous duty operation!
If you walk out to the airplane and discover that the battery is dead (perhaps because the battery switch was left on), resist the temptation to hook up a battery cart or APU for starting. If you go flying with a dead battery, one of two things may happen: (1) you won't be able to get the battery on-line, or (2) the alternator will charge the battery too rapidly and damage or destroy it. The only good solution for a dead battery is to hook it to a battery charger and then go have a leisurely breakfast or lunch while it charges for an hour or two.
Once the engine starts, check the ammeter for excessive charging current, and check the voltmeter (if you have one) for proper bus voltage (14 or 28 volts) before turning on any of your expensive avionics. If the ammeter is showing an excessive charge or the bus voltage is low, you might have a stuck starter contactor or a shorted battery. In either case, you want to shut down and call a mechanic.
Taxi and runup
One the engines are running and it's time to taxi out, perhaps the most common mistake I see pilots make is failing to lean the engine properly. The Continental fuel-injection system is set up with a very rich idle mixture in order to facilitate cold starts. Taxiing with the mixture full-rich is like driving a car with the choke on...the engine is literally awash in excess fuel. This often results in fouled plugs, contaminated exhaust valves, and fuel dilution of the oil film on the cylinder walls.
As soon as the engine starts to warm up (less than a minute after start in temperate weather, perhaps a couple of minutes if it's really cold), you should lean the engines for taxi. Retard the mixture control until the engine starts to stumble, then enrichen a bit for smooth idle. You cannot hurt the engine by leaning it at these power settings...but you will hurt it if you taxi around with the red knob full forward.
Another common taxi mistake I often see is carrying too much power and riding the brakes to control taxi speed. This is especially common in twins, because with two engines running they have considerable idle thrust. Rather than using 1,000 or 1,200 RPM and having to ride the brakes, try 800 or even 700 RPM for taxi. This is especially important if you're taxiing downwind (which is normally the case if you think about it) or taxiing on a downgrade.
Incidentally, big-bore Continentals are supposed to idle smoothly at 600 RPM, and they will if everything is properly adjusted. If your engine runs rough at low RPMs, something's wrong. Ask your mechanic to check your low unmetered fuel pressure, idle mixture, and magneto timing.
Once in the runup area, do a runup at 1700 RPM (unless you're flying a GTSIO). In twins, I recommend checking each magneto individually (rather than two at a time). You're looking for a significant RPM drop as each mag is shut off, and the drops should be approximately the same (within 50 RPM) for the right and left mag. Incidentally, a too-small mag drop (say, less than 50 RPM) is as bad or worse as a too-big mag drop (say, more than 150 RPM). And zero mag drop usually means you have a broken P-lead and a dangerous "hot mag" condition, which you'll want to get fixed ASAP.
My preference is to leave the mixture leaned for idle when I do my runup. Others prefer to go full-rich for runup, and then re-lean for idle if there's a delay. Pick your poison.
In cold weather, make sure the engine is fully warmed up before you take off. Oil temperature and CHT should both be well into the green and stabilized. You'll damage your cylinders if you apply takeoff power while they're too cold. And if you're turbocharged, the wastegate actuator and controller won't work worth a damn if the oil is too cold.
Takeoff is an especially hazardous phase of flight, and that goes double if you're flying a piston twin. My first step in every takeoff is to recite what I call my "killer items" checklist. It's a short memorized list of items that could to kill me if I get them wrong. In my airplane (a Cessna T310R), they include:
- boost pumps low
- pitot heat on
- wing flaps up
- trim set for takeoff
- fuel selectors mains
- props high RPM
- mixtures full rich
and although it may be stretching things to call them "killer" items, my checklist also includes:
- landing/taxi lights on
- transponder on
Note that the use of boost pumps for takeoff varies from one installation to another, so be sure to check your POH. The same is true for the use of takeoff flaps. In general, I prefer not to use takeoff flaps in twins because I don't want the airplane to get "light on its feet" before reaching Vmc. In most singles, however, I prefer to use takeoff flaps if they are authorized by the POH.
Having completed the pre-takeoff checklist, it's time to pour on the coals. This is a very significant thermal event for the powerplant, which goes from idle power to maximum rated power in a matter of seconds. For optimum engine longevity, we want to minimize the gradient of this thermal event by throttling up as slowly as possible. However, we are constrained in how long we can take to do this by the length of the available runway.
My procedure is to taxi into position and hold, brakes set, and then advance the throttles very slowly to about 50% power (MP in the high teens or low twenties for most of these engines). The purpose of this exercise is to "buy time" without consuming runway. On the other hand, I prefer not to throttle up beyond 50% power with the brakes set because it's tough on the prop tips and tail feathers, and because there's very little cooling air passing over the engine.
Once at about 50% power and having verified that all engine gauges are in the green (and in a twin, no significant "splits"), I release the brakes and continue throttling up very slowly as the aircraft accelerates down the runway. If the runway is relatively long, I'll try to advance the throttles just fast enough to reach full takeoff power as the airplane achieves rotation speed. If the runway is shorter, I'll throttle up faster.
The takeoff roll is a very busy time in terms of pilot workload. Primary focus should be looking out the windshield and tracking the centerline. But it's also necessary to scan the flight instruments and engine instruments briefly as the takeoff roll progresses. Fairly early in the roll, I look for the airspeed needle to come off the peg, at which point I make a verbal "airspeed alive" call to myself. In a turbo, I check the manifold pressure to make sure there's no overboost. Finally, just prior to rotation, I make another full scan of the engine gauges (MP, RPM, fuel flow, oil pressure, oil temperature, and CHT) to make absolutely sure everything's okay before committing the airplane to fly.
I believe that it's very important to perform every takeoff with a mind-set that is "spring-loaded to abort." If anything looks, feels, sounds or smells wrong, don't try to troubleshoot it on the roll...just abort the takeoff and sort it out during the taxi-back.
Most pilots don't practice rejected takeoffs (RTOs) very often, if ever. We do a lot of them in the simulator when I train at FlightSafety or Simcom. If you don't do simulator-based recurrent training, then make sure your CFI has you abort a takeoff of two during your flight review or instrument proficiency check.
Turbine pilots are taught that its usually safer to take off with a problem than to try to abort and run the risk of running off the end of the runway. But in a piston aircraft single or twin the opposite is usually true: it's almost always safer to reject the takeoff.
Once the gear is up and the airplane is climbing through pattern altitude (1,000' AGL), it's time to reduce to cruise-climb power. (In a normally aspirated airplane, it's acceptable to remain at full-throttle and to let Mother Nature do the job; in a turbo, the pilot has to make the power reduction.)
Normal cruise-climb in a Continental-powered airplane is normally 75% power. In many aircraft (including most Cessna singles and twins), this occurs at top-of-the-green manifold pressure and top-of-the-green RPM. Don't fixate on the engine gauges while making this power reduction, especially in IMC. Learn to do it mostly by feel and sound, with just occasional quick glances at the power instruments.
Once the power reduction has been made, its time to lean the engine for climb. Some airplanes have a recommended cruise-climb fuel flow marked on the face of the fuel flow gauge (e.g., with a blue triangle). If yours doesn't, look up the correct fuel flow in the POH and mark it on the gauge or put it on a placard next to the gauge.
If your primary CFI taught you not to lean below 5,000 feet, please purge that idea from your cortex right now. You should always lean the engine whenever it's operating at 75% power or less, regardless of altitude. You should be leaned for climb, cruise, descent, landing, and taxi. The only time you should be full-rich is for start, takeoff, and go-around.
In a normally-aspirated airplane, you'll have to add throttle every 1,000 feet or so to maintain cruise-climb power. With a turbo, the automatic wastegate system should hold your manifold pressure more-or-less constant as the aircraft climbs. Some fall-off is normal, however, especially if you use single-weight oil. So check the MP occasionally and re-tweak the throttles as necessary.
Check the CHT gauge from time to time during the climb and try to keep the CHT at or below about 400°F (certainly no higher than 425°F). Cooling can be a problem when climbing out in very hot weather or to high altitudes (where indicated airspeed is low and the engines get less cooling), and that goes double for turbocharged airplanes. If you notice your CHTs getting warmer than this, the best way to bring them down is by trimming nose-down to trade reduced rate-of-climb for increased airspeed. Cowl flaps aren't terribly effective at low airspeeds, particularly at high altitudes. Enrichening the mixture can help bring down CHTs, but at the expense of contaminating the engine with unburned byproducts of combustion. Increasing climb airspeed is usually your best bet.
Upon reaching cruise altitude, level off by rolling in nose-down trim or engaging altitude-hold on the autopilot. Don't be in a hurry to reduce power...leave it at 75% cruise-climb power until the airspeed has accelerated to cruise speed and stabilized. Close the cowl flaps if they were open during the climb.
Although big-bore Continentals are generally rated for continuous cruise at 75% power, I recommend reducing to a more conservative cruise power setting between 55% and 65% power. 55% provides near-optimum fuel economy, while 65% provides a good compromise between fuel economy and airspeed.
A major advantage of using cruise power of 65% or less is that the engine may be leaned much more aggressively at such power settings. TCM authorizes leaning right to peak EGT at power settings of 65% of less, and I recommend doing exactly that. In addition to saving fuel, you'll be operating with cleaner combustion and that will pay dividends in engine longevity.
I also am a big fan of cruising at low RPMs and high MPs rather than the other way around. If your engine will run smoothly at bottom-of-the-green RPM and top-of-the-green MP, that's an excellent place to operate them for cruise. (For the TSIO-520s in my Cessna T310R, that's 30" MP at 2100 RPM, and that's exactly the setting I use at altitudes up to 13,000' or so.) If your engines don't feel smooth at bottom-of-the-green RPM, experiment to find the lowest RPM at which they do feel smooth, and cruise at that.
If you're flying a turbo and climbing up to the Flight Levels, you'll probably find that you can't use bottom-of-the-green RPM without "bootstrapping" (a condition where the wastegate is completely closed, the turbocharging system is operating unregulated, and significant MP excursions become evident in flight). The cure for such boostrapping is to increase RPM in small (50 RPM) increments until MP stabilizes. If you still have difficulty stabilizing MP at high altitudes, you may have an induction or exhaust leak or some other engine problem. The best way to diagnose this is to perform a "critical altitude check" as outlined in the service manual. Critical altitude for a turbocharged airplane is the maximum altitude at which the engines can develop full takeoff power. For my T310R, that's supposed to be 16,000'; for a T210 or 340 or 414, it's 20,000'. If you can't get full takeoff power at the airplane's rated critical altitude, then you have a problem that needs to be found and fixed.
Another high-altitude problem is that you may exceed the turbocharger's turbine inlet temperature (TIT) limit before reaching peak EGT. Cessna did not install TIT gauges in most of its turbocharged airplanes, but many have aftermarket gauges (like GEMs or JPI 700s) with a TIT readout. Published TIT redline is usually 1650°F but for optimum turbocharger and exhaust system life, it's a good idea to limit TIT to 1600°F or less. If your TIT is higher than that, you can either reduce cruise power or run richer; I'd recommend the former.
If an engine suddenly starts running rough at Flight Level altitudes, you may be experiencing high-altitude misfire. Try descending a few thousand feet. If the problem goes away, the diagnosis is confirmed. If you experience high-altitude misfire, ask your A&P to adjust your spark plug gaps to the low end of the allowable range (generally 0.016") and to clean the distributor caps of your magnetoes. Those simple steps almost always cure the problem.
One more comment about in-flight engine trouble: If you experience a sudden unexplained loss of manifold pressure in a turbocharged airplane, it might be an indication of an exhaust system failure and should be grounds for making an immediate precautionary landing at the nearest airport. In a turbocharged twin, you should also give serious consideration to doing a precautionary engine shutdown.
As you approach your destination, your engine needs to be cooled down gently for maximum longevity. This is especially important if you're flying a turbocharged aircraft. The key is proper descent planning and temperature management.
For most high-performance singles and twins that cruise in the 200-knot range, you can figure your descent airspeed will be about 4 NM per minute, and the maximum comfortable descent rate is about 1000 FPM. Consequently, if you have X thousand feet to descend, you should start down about X minutes or 4X miles out from the traffic pattern or approach gate. (The "approach gate" is an imaginary point about two miles outside of the FAF on an instrument approach at which the airplane should be stabilized in approach configuration.) If you're flying a turbo and descending from FL250 into a sea-level airport, you'll need to start down at least 100 NM and 25 minutes out. So plan ahead!
You also need a plan for cooling down the engine. The one I use is to allow one minute for each 1" of MP that I need to reduce to get from cruise power to approach power. In my airplane, normal cruise is at 30" MP and approach is at 20" MP. So I have 10" to lose and need to allow 10 minutes. Consequently, I make an initial power reduction of about 2" MP when I'm 10 minutes (or 40 miles) out from the traffic pattern or approach gate, and continue to make additional 2" MP reductions every two minutes thereafter until I reach my approach power setting. I recommend using a timer for this. (In a normally-aspirated airplane, you probably need to lose only 5" of MP or so, so you need only 5 minutes to do it.)
Don't let a "slam dunk" request by ATC seduce you into making an abrupt power reduction that could damage your engine. The controller probably doesn't realize that what he's asked could be hazardous to your engine's health (the turbine guys he usually works don't seem to mind a bit), and you can bet the FAA won't pay for your cracked cylinders. Just tell ATC "unable" and offer to accept a delay vector or a turn in holding while you cool down those high-strung engines.
How about mixture control during descent? Turbocharged Continentals use an altitude-compensating fuel pump that generally allows you just to leave the mixtures where they were at cruise and not mess with them. I recommend not enrichening the mixtures for descent unless the engines start running rough. For normally-aspirated engines, you'll have to gradually enrich as you descend, but don't enrich so much as to make the EGTs drop too much. Stay as lean as you can consistent with smooth engine operation.
Follow the recommended procedure in your POH with regard to use of the electric boost pump for landing. In some aircraft (including my T310R), the pumps should be set to "low" prior to landing. But other models do not use the boost pump for landing, so be sure to check your POH and do what it instructs with respect to boost pump usage.
Every POH I've ever read instructs you to advance the mixture to full-rich on final prior to landing. Don't do it! Pouring cold fuel on a hot cylinder head simply can't be a good thing for cylinder longevity. I recommend leaving the mixture leaned out for landing and taxi. I also recommend setting the props to top-of-the-green RPM, not shoving them full forward the way the POH instructs. Of course, if you have to make a go-around or a missed approach, don't forget to advance the mixture and prop controls before throttling up to full power.
A turbocharger should be given the opportunity to cool down (and spin down) at idle for 3 to 5 minutes before shutting down the engine and thereby cutting off the flow of oil to the turbocharger. In many cases, the landing roll and taxi-in take enough time that no additional cool-down is required. If this is not the case (e.g., if your parking spot is close to the point you turned off the runway), let the engine idle for a few minutes at your parking spot before you pull the mixtures to idle cutoff.
Before shutting down the engine, make sure that the boost pump is turned off. Otherwise, you might inadvertently pump fuel into one or more cylinders and create a damaging hydrostatic lock. It's also a good idea to make sure that the avionics master switch and other major electrical loads (like landing and taxi lights and pitot heat) are turned off prior to engine shutdown.
Some pilots are in the habit of throttling up just prior to engine shutdown, thinking that it will somehow help prevent fouled spark plugs. It's not a good idea, and is especially rude if there are people or airplanes behind you. If you keep the mixture leaned during taxi operations as I've recommended, you won't have plug fouling problems.